[ARM] MVE VMOV immediate handling
This adds some handling for VMOVimm, using the same method that NEON uses. We
create VMOVIMM/VMVNIMM/VMOVFPIMM nodes based on the immediate, and select them
using the now renamed ARMvmovImm/etc. There is also an extra 64bit immediate
mode that I have not yet added here.
Code by David Sherwood
Differential Revision: https://reviews.llvm.org/D63884